The move came amid what Western governments, Kremlin foes, and activist groups including AI itself say has been a persistent clampdown on civil society organizations -- particularly those with foreign ties -- during President Vladimir Putin's third term.
Sergei Nikitin, head of AI's office in Russia, told RFE/RL outside the sealed premises in central Moscow that, when his colleagues arrived for work on November 2, they found that the door had been forced open during the night and the locks changed.
"We have to work out what the reasons for this were," said Nikitin, who said AI has leased the premises from the Russian capital's State Property Department for more than 20 years and has always paid rent on time.
"Because this is the style of bandits -- when they change your locks and seal your office even though you've paid," he said. "We need to get to the bottom of this. Maybe it was all a mistake."
Russian Internet and TV channel Dozhd later cited the Moscow property department as saying that the premises were now "free of contract relations," suggesting that the lease had expired, and that the locks had been changed.
The seal on the door warned against "unsanctioned entry," instructed staff to contact the owner, and included a phone number.
"We tried to call the number on the seal. However, we have not been able to get through. We have now been trying for three hours, if not more. No one is picking up the phone," Nikitin said.
"This is not a question of debt or not paying. There are other reasons at play here," Nikitin added. "And we can only guess at the motivations for what I would call unfriendly and even violent action on the part of the Russian authorities."
"Given the current climate for civil society work in Russia, there are clearly any number of plausible explanations, but it's too early to draw any conclusions," John Dalhuisen, Amnesty International's Europe Director, said in a statement. "We are working to resolve the situation as swiftly as possible and very much hope there is a simple administrative explanation for this setback to our work."
Putin's spokesman, Dmitry Peskov, told journalists on November 2 that he was unaware that Amnesty International's office had been sealed. Moscow city officials could not immediately be reached for comment.
Kremlin critics have accused Putin of a carrying out a campaign of pressure against civil society and human rights organizations in recent years, frequently targeting groups that are foreign or receive funding from abroad.
Putin, in power as president or prime minister since 1999, has long accused Western governments of using rights groups to spy on Russia and undermine his rule.
Putin has stepped up criticism of the West since returning to the Kremlin for a third term in 2012, after he weathered street protests that he accused the United States of fomenting. Since then, he has signed laws authorizing the government to brand foreign-funded organizations "foreign agents" and to blacklist foreign-based groups as unwelcome in Russia.
The AI office was raided by tax police in 2013, in what the authorities said was an "audit," but Nikitin told RFE/RL on November 2 that the organization has not faced overt government pressure recently.
"I cannot remember any serious signs of enmity at all," he said.
However, AI frequently criticizes Russia's treatment of activists, dissenters, and prisoners. On November 1, the organization called for the immediate release of Ildar Dadin, an imprisoned activist who alleged that he and other inmates have been tortured.
In its annual report on human rights worldwide, released in February, it said that freedoms of expression and peaceful assembly continued to be severely curtailed in Russia in 2015, and that attempts to silence civil society were made with the aid of the repressive use of vague national security and anti-extremism laws.
"We have seen more crackdowns on the freedom of expression, on the freedom of peaceful assembly, on the freedom of association," Denis Krivosheev, Amnesty's deputy program director for Europe and Central Asia, told RFE/RL at the time. "NGOs have been under pressure for a number of years and [the pressure] has massively increased over the last year."